A general microphone converts a displacement of the diaphragm vibrating in response to a sound pressure into an electric signal by using a coil or capacitor. A microphone has been proposed which is of the type that vibration of the diaphragm is converted into an electric signal by utilizing light. A known microphone of such a light use type will be described with reference to FIGS. 15 to 17. In a light detection type microphone 70 shown in FIG. 15, a diaphragm 72 mounted on the front side of a case 71 has a mirror surface on the inner side and vibrates back and forward in response to a propagating sound wave. The case 71 having a mirror surface of the inner wall has the diaphragm 72 mounted on the front opening end and receiving a sound pressure and a partition plate 75 for leaving a gap 76 between the top of the partition plate 75 and the diaphragm 72 and partitioning the inside of the case 71 excepting the gap 76 into two rooms. A light emitting element 73 and a light receiving element 74 are disposed in the partitioned rooms opposite relative to the partition plate 75. Light emitted from the light emitting element 73 is reflected at the inner mirror surface of the diaphragm 72, passes through a gap 76 and enters the light receiving element 74. A condenser lens 78 is disposed on an optical path between the light emitting element 73 and diaphragm 72 and converges light at a predetermined position of the diaphragm 72. A condenser lens 79 is disposed on an optical path between the diaphragm 72 and light receiving element 74 and converges light reflected by the diaphragm 72 at the light receiving element 74. The size of the gap 76 changes with a vibration displacement of the diaphragm 72 so that a light reception quantity of the light receiving element 74 is a function of a vibration displacement quantity of the diaphragm 72. An electric signal related to a sound pressure can thus be generated from a light reception quantity of the light receiving element 74.
FIGS. 16 and 17 are a schematic diagram and a detailed diagram showing a light detection type microphone 83 utilizing light according to another conventional technique. A monitor photodiode 85 detects a quantity of laser light irradiated from a semiconductor laser 84. A laser APC 86 controls an output of the semiconductor laser 84 in accordance with an output of the monitor photodiode 85, to thus maintain constant a radiation quantity of the semiconductor laser 84 during operation. A diaphragm 89 disposed in front of the semiconductor laser 84 has a mirror surface on the inner side and vibrates in response to a sound pressure. A laser beam from the semiconductor laser 84 passes through an objective lens 90 and becomes incident upon the diaphragm 89. The reflected light passes through the objective lens 90 and becomes incident upon a diaphragm displacement detector diode 91 which detects a light quantity. Referring to FIG. 17, each element shown in FIG. 16 excepting the diaphragm 89 is housed in a case 93. The peripheral of the diaphragm 89 is supported by the front wall of the case 93. The case 93 has a plurality of communication holes 94 for making the inner surface side of the diaphragm 89 communicate with an external. The semiconductor laser 84 and diaphragm displacement detector diode 91 are mounted on a mount substrate 96. A laser beam from the semiconductor laser 84 is irradiated to the inner mirror surface of the diaphragm 89 via a reflected light flux splitting element 97, the objective lens 90 and an achromatic transparent lid 98. The reflected light becomes incident upon the diaphragm displacement detector diode 91 via the achromatic transparent lid 98, objective lens 90 and reflected light flux splitting element 97. The achromatic transparent lid 98 prevents a sound pressure from propagating via an opening over which the lid is mounted. A focussing actuator 99 controls the position of the objective lens 90 along the axial direction by utilizing known focus servo control to be used by a compact disc (CD) player or the like. More specifically, the position of the objective lens 90 along the axial direction is controlled in accordance with the frequency components, for example, lower than 20 Hz (low frequency components lower than audible frequency) of a focus error signal detected with the diaphragm displacement detector diode 91. Regardless of vibration of the diaphragm 89, the focus of the laser beam can be positioned on the diaphragm 89. A sound pressure in the audible frequency range can be detected by deriving the focus error signal at 20 Hz or higher from the diaphragm displacement detector diode 91.
The light detection type microphone 70 shown in FIG. 15 is associated with the following problems. It is difficult to adjust the gap 76 in a sound pressure light reception region where a linear relation between a sound pressure and a light reception quantity can be obtained. A light reception quantity of the light receiving element 74 is likely to vary because of a variation in a divergence angle of light emission of the light emitting element 73 and a variation in a direction of the light emitting element 73. The light detection type microphone 83 shown in FIGS. 16 and 17 is associated with the following problems. Although a variation in the characteristics of each element and a variation in the mount position of each element can be suppressed, the layout of components becomes long along the axial direction and a compact layout is difficult. In order to improve the directivity of the microphone, it is necessary to react a sound pressure from a sound source also with the inner surface of the diaphragm 89. However, if the diaphragm 89 is positioned near at the objective lens 90 in order to make compact the light detection type microphone 83, the objective lens 90 hinders the propagation of a sound pressure to the inner surface of the diaphragm 89, resulting in a degraded directivity. It is necessary for the light detection type microphone 83 to apply a spot of a laser beam to the diaphragm 89 and detect the reflected light. It is therefore necessary to maintain always clean the inner mirror surface of the diaphragm 89. However, the inner mirror surface of the diaphragm 89 is likely to be blurred because of chemical reaction of chemical gas contained in the atmospheric air at a small quantity and because of attachment of dust.